Structural Regulation And Sodium Storage Properties Of Manganese-Based Cathode Materials

Sodium ion battery has become the most potential new system in the field of large-scale electrochemical energy storage technology because of its rich resources and low cost.The research and development of high-performance cathode materials is the key to improve the energy density of Sodium ion battery and accelerate its practical application.Sodium layered transition metal oxide is one of the significant cathode materials for sodium ion batteries,with high adjustable of chemical composition and sodium storage,which bring the advantages of high capacity,good stability,and low cost.However,due to the large radius of sodium ions and complex phase transition process,the bulk phase structure and surface structure of these materials are easy to be unstable,and the cycle stability is poor.In this paper,the strategy of element doping and sodium content regulation is adopted to optimize the structure of manganese base material and improve its sodium storage performance.The main research work is as follows:（1）A series of orthorhombic P′2 phase Na_0.67Cu_xMn_1-xO₂（x=0,0.05,0.1,0.2）materials were prepared by combining copper doping and phase structure regulation.DFT theory calculation and in-situ XRD revealed the structural evolution and the phase transition process in the electrochemical process of doped and undoped materials,the results show that this strategy can effectively suppress the structural distortion and multiple phase transitions of P′2 Na_xMn O₂ materials during charge and discharge process.Meanwhile,the kinetic analysis show that the sodium ion diffusion rate and stability of copper doped materials are higher than those of undoped materials.The cathode materials P′2 Na_0.67Cu_0.1Mn_0.9O₂ has a discharge specific capacity of 222.7 m A h g^-1 at 0.1 C(1 C=200 m A g^-1),and with capacity retention of 76%at 5 C after 300 cycles.（2）On the basis of copper and nickel doping,a series of hexagonal P2phase Na_xNi_0.15Cu_0.1Mn_0.75O₂（x=0.5,0.6,0.7,0.8,0.9）materials were prepared by adjusting the sodium content.Copper and nickel doping can improve the redox potential,and the regulation of initial sodium content can improve the cycle stability of the material.It is found that even when the stoichiometric ratio of transition metal elements is the same,the redox potential of these materials is significantly different,which is caused by the change of local structure caused by different interlayer sodium content and lattice oxygen.The cathode materials Na_0.7Ni_0.15Cu_0.1Mn_0.75O₂ has a discharge specific capacity of 186 m A h g^-1 at a current density of 0.5 C(1C=100 m A g^-1).